A pair of Democratic and Republican lawmakers have joined forces to pressure the Federal Reserve to get tougher on Wall Street banks by requiring financial giants to maintain bigger capital cushions that can help protect them against losses.

In a letter to Federal Reserve Chairman Ben Bernanke, Sens. Sherrod Brown (D-Ohio) and David Vitter (R-La.) warned that a Federal Reserve proposal released in December on capital standards “misses a huge opportunity to address the too-big-to-fail issue.”

The senators, who are both members of the Senate Banking Committee, wrote that they want the Fed to go beyond the minimum capital requirements laid out in an international agreement on capital, known as Basel III, and force the largest U.S. lenders to meet a tougher standard.

“The surcharge on the megabanks should be high enough that it will either incent them to become smaller or will help to ensure they can weather the next crisis without another taxpayer bailout,” the senators wrote in a letter dated Aug. 6.

The letter is further evidence that pleas from large banks, such as JPMorgan Chase, that new capital requirements go too far are falling on deaf ears across the political spectrum.

Democrats have pushed the issue as a way to further crack down on large banks while Republicans have embraced the idea as a more elegant solution to fears about financial instability than the myriad of rules contained in the 2010 Dodd-Frank financial oversight law.

“Greater capital is essential to withstand inevitable losses in the banking industry,” Brown and Vitter wrote.

In the aftermath of the global financial crisis, financial regulators from the world’s leading economies have sought to impose tougher capital standards on banks by making institutions fund themselves more through equity than debt so that they can better absorb losses and, in turn, make the financial system more stable.

The Basel Committee on Banking Supervision, of which the United States is a member, reached an international agreement in 2010 on the Basel III framework for enforcing stricter capital standards on all banks. It will be phased in between 2013 and 2019.

Late last year, this group of regulators also agreed that the world’s 29 largest banks, including eight in the United States, should have to meet an even tougher standard.

In December, the Federal Reserve released a set of proposed rules that included new capital requirements but it did not address the issue of the added charge for the largest banks, something Fed officials said the central bank will do in the coming months.

Brown and Vitter made clear in their letter that the tougher standards should be focused only on the largest banks, such as the eight tapped by the Basel committee, and not on smaller regional banks they said pose less of a threat to the financial system.

“U.S. regulators must focus their efforts to impose enhanced capital requirements on the largest, most complex financial institutions, and not the smaller, regional institutions that engage in traditional banking services and whose systemic footprint is limited or inconsequential,” they said.